The present invention relates generally to the field of superconducting coils and, more particularly, to a protective link for protecting a superconducting coil.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Superconductivity is the property of certain materials at cryogenic temperatures approaching absolute zero to carry electric currents without significant power dissipation. Low-temperature superconductors, which operate at temperatures below 10 K, are “ideal” in the sense that they have zero dc resistivity and hence produce zero power dissipation when operated within characteristic current and magnetic-field limits. High-temperature superconductors (HTS), which exhibit superconducting characteristics at liquid Nitrogen temperatures (77 K) and above are not ideal but rather are characterized by extremely low voltage drop, (again when operated within characteristic current and magnetic-field limits) and thus produce extremely low power dissipation as compared to conventional conductors under the same operating conditions. Because these high-temperature superconducting materials may be used more readily, the range of applications for their use has increased dramatically. High-temperature superconductors have applications in medical imaging systems, motors, generators, high-field magnets, etc.
The voltage-drop in HTS wire, and correspondingly across an HTS coil, is a highly non-linear function of the coil current as well as the coil temperature and magnetic field. As the coil current is increased the power dissipation will increase and at some point will exceed the capacity of the cooling system to achieve an equilibrium condition in the coil. Under such a condition, the temperature of the coil, as well as the coil voltage drop and power dissipation, will be observed to increase without apparent limit and, if this condition is maintained, will rise to the point that the coil may be damaged or destroyed. When this condition occurs, the coil is said to be undergoing a quench and it is typically desirable to take preventative action before damage occurs.
Quench can be initiated by a variety of circumstances. As described above, it can be initiated simply by operating an HTS coil at currents in excess of a maximum operating limit corresponding to normal coil operating conditions. Alternatively, an HTS coil may quench if the cooling system fails when the coil is operating at what would otherwise be an acceptable current level. In this case, the cooling system failure will result in a higher-than-expected coil temperature, voltage drop, and power dissipation.
Independent of the initiation event, it is necessary to detect the onset of a quench and to take preventative action. Various schemes based upon coil voltage, coil current, and other winding parameters have been devised to detect the onset of a quench event. Based upon the output of these detectors, HTS-coil current supplies are designed to shut down and to de-energize the coil so as to avoid coil damage.
Protection of an HTS coil is analogous to the protection of many electrical systems. For example, electric motors are frequently protected by thermally-operated mechanical disconnects. However, in most cases, there is a back-up system, consisting of a fuse or circuit breaker, selected to operate in case the primary protection system does not operate.